I just watched a 2008 episode of Holmes on Homes. This is an HGTV show in which Holmes goes to different towns to fix repairs done by shotty contractors. They had a 200 amp panel, the entrance cable came in the top, so did the branch circuits. Holmes claimed this was a code violation. He pulled the entire panel out and remounted it horizontally, keeping the entrance cable wire seperate from branch circuits. (It was a Square D- QO panel). I have always believed that the panel is to only be mounted vertically????